Voltage comparator



Oct. 21, v1958 2,85 7,568

K. W. HERING ETAL VOLTAGE COMPARATOR Filed July 22. 195s 4 sheets-sheet 1 BY obefl/lane oct. 21, 195s K. WHERNG Em 2,857,568

- -voLTAGE coMPARAToR 4 sheets-sheet 2` Filed July 22. 1953 T0 NULL'.

o a INDICATOR ATTORNEYS Oct. 21, 1958 K. w. HERING ETAL 2,857,568

VOLTAGE coMPARAToR Filed July 22. 1953 4 sheetssheet 3 [Kaff W/e z' obefZ/falle ATTORIVEYS l [g i United States Patent() VOLTAGE ComARAroR Karl W. Hering, Ridgefield, and Robert F. Kane, Jr., Norwalk, Conn., assignors to The Perkin-Elmer Corporation, Norwalk, Conn., a corporation of New York Application July 22, 1953, Serial No. 369,688

14 Claims. (Cl. 324-99) This invention relates to a device for determining ratio and phase relationships of two voltages, and more particularly for the measurement of the ratios of the in-phase and quadrature components of a voltage derivedl from an A. C. source to the in-phase component of a larger voltage derived from the same source.

For example, for two signals in an electronic computer, these ratios are determined by using the source voltage as the phase reference. This reference is modiiied by attenuation and phase reversal, if necessary, to a voltage equal in magnitude but opposite in phase to the in-phase component of the larger of the two voltages being measured. The second voltage is then compared against this modied reference voltage, with its in-phase and quadrature components ratioed thereto.

A common source for the instrument and the signals under study is necessary to establish phase reference during measurement. Frequency and 'phase changes are zero and any amplitude changes affect both the numerator and the denominator of the ratio of the signal voltages in the same proportion. Hence, the true ratio is undisturbed by line changes.

The invention will be understood by reference to the following description considered in connection with the accompanying drawings in which:

Figure 1 is a block diagram of a manually operable instrument in accordance withV the invention;

Figure 2 is a circuit diagram, partly schematic and partly in block form, of a second embodiment of the invention;

Figure 3 is a circuit diagram, partly schematic and partly in block form, of a third embodiment of the invention;

Figure 4 is a circuit diagram, partly schematic and partly in block form, of a fourth embodiment of the invention;

Figure 5 is a schematic circuit diagram of the instrument in Figure 1; and

Figure 6 is a front View of a panel layout for the instrument in Figures l and 5.

Referring now to Figure l, a summing device 1 has an output 2 leading to the input of a suitable null indicator and three inputs, one each from a test probe 3, inphase channel 4 and quadrature channel 5. Channels 4 and 5 are connected in parallel to a source 6 having one side power grounded.

The null indicator is not shown, but any suitable voltage sensitive device, such as an oscilloscope, a servo system, or an amplifier-phone combination, may be used. An oscilloscope is adopted in the description which follows, with output 2 establishing the Y axis and the reference voltage the X axis.

In-phase channel 4 includes, in series, an isolation de vice 7 which provides an isolated ground system Within the instrument and permits reversal of the phase-of voltage 6, a Calibrating attenuator 8 adjustable by dial 9, a precision attenuator 10 adjustable by dial 11, and

means for reversing the phase, as by a gang switch 12 to be more fully described below. It may be desirable to include also a scale changer 13Y for expanding the scale associated with dial 11. v

Quadrature channel S comprises, in series, phase shifter and isolation device 14 and xed attenuator and isolation device 15, selectable through gang switch 12, and an attenuator 16 adjustable by dial 17, together with means for phase reversal as bythe signal grounded center tap 18 of attenuator 16. v

Gang switch 12 also interrupts the circuit from'probe 3 to permit the probe to be disconnected and to ground the input to the summing impedance. v

The instrument may be used for a `rapid and very accurate determination of the ratios of the in-phase and quadrature components'of a voltage derived from source 6 to the in-phase component of another voltage also derived from source 6, by employing source 6 as a reference voltage. To make these measurements, dial 11 of precision attenuator 10 is rst adjusted to a convenient ratio denominator which may be its maximuml setting of 100, for instance. With the signal ground system of the instrument tied in with the signal ground system of the unit to be tested andthe source input through line 19 and attenuator 19a established as the X axis reference of the oscilloscope, test probe 3 is touched to the tap of the larger signal voltage, called V1. Through gang switch 12, the signal V1 4is impressed upon the summing device 1 and the resultant signal israpplied to the Y axis of the oscilloscope.v Isolation device 7 isolates a Voltage in channel 4 either in-phase or 180 out of phase with the source, depending upon ,the setting of gang switch 12, with the result that the inphase component of V1 (called Vpl) whether in-phase or out of phase with the reference voltagevmay be nulled by turning dial 9 of Calibrating attenuator 8 until the magnitude of the isolated voltage is matched to that of Vm. In order to ascertain the exact null it may be desirable or necessary to eliminate the quadrature component (called'Vql) in `summing device 1, and this may be `accomplished by channel 5. There 90 phasel shifter and isolation device 14 establishes an isolated voltage lagging the reference voltage by 90, which, if Vql is leading 90, may be matched to that of Vql by turning dial 17 of lattenuator 16 until a null is obtained. In practiceV dials 9 and 17 may be adjusted simultaneously to null both components of V1. Should Vql lag the'reference voltage by 90, then the phase of the isolated voltage inchannel 5 will have to be` shifted 180 so as to be reversed, as may be accomattenuator 16, for purposes of nulling. At the null point there is effectively set into channel 4 an attenuation factor equal to the ratio of V91 to thereference voltage. The output of channel 4 is a voltage, called the modiied reference voltage, equal in magnitude to Vm. Y'

Now by touching probe 3 to the second signal Vgl, and nulling its in-phase component 'Vp2, Withl dial 11 of precision attenuator 10 and nulling its quadrature component Vqz, with dial 17 as before, the reading on dial 11 indicates directly the ratio Vpz/Vpl, since vdial .11 initially was at its maximum setting. By Calibrating dial 11 from 0 to 100, a percenta'gereading may be had.

It may be necessary tov reverse the direction of the phase voltage of equal magnitude and opposite phase with `re.-

spect to Vqg (the position of dial 17 Withrfespecttto f ground '18 is likewise indicative of the phase of Vqz). The ratio Vqz/Vpl can then be measured by positioning gang switch 12 to disconnect the signal summing input from the probe and signal .grounding it. In -this position gang switch 1'2 also disconnects the 90 phase shifter .and isolation device 14 from attenuator 16 and in its place connects fixed attenuator and isolation device `15 which :furnishes to attenuator 16 a signal equal in amplitude to that of phase shifter 14 but in phase with the source 6. The attenuated in-phase output of channel 5 may therefore be nulled by appropriate adjustment of .dial 11. The reading on dial 11 is then equivalent to the ratio Vqz/Vpl. The phase of the isolated voltage in .channel 4 maybe reversed with gang switch^12 if necessary to .obtain a null, and similarly its position indicates the quadrant phase of Vqz.

A circuit for accomplishing the foregoing measurements Ais shown in Figure 5. There the instrument is .connected across terminals of the source 6 from which the two signals to be compared are also derived. This `input also establishes through line 19 and attenuator 19a the X axis reference for the oscilloscope, with a resistor 19b and a tap G to signal ground. The Y .terminal of the oscilloscope is connected directly to out- ;put 2 of summing device 1.

Summing device 1 receives input from test probe 3 through a resistor 20, from channel 4 through a resistor 21, and from channel 5 through a resistor 22. All .input ylines to the summing device, its output line, and ,the .test probe lead are shielded, with shields power grounded. Channel 4 employs as an isolation device 7 a transformer suitably corrected for internal phase shifts,

with its primary Winding connected across the reference :phase 6 and the secondary connected to a two-pole, fouri position gang switch 12 for phase reversal. The switch output is impressed upon a Calibrating attenuator 8 Which, in this embodiment, is a two-step device having a variable .inductance 23 signal grounded on one side and adjustable .with dial 9 for coarse calibration, and a variable resistor f24in series with inductance 23 and adjustable with a V.dial 25 to a finer calibration. A resistor 26 is shunted from the high side to the arm of inductance 23 for load correction. Lower pick-up is experienced by use of a variable auto-transformer instead of a rheostat for calibrating attenuator 8 because of the lower output impedance.

The output of calibrating attenuator S is connected to rprecision .attenuator 10 which here is shownto be a variable -resistor 27, suitably signal grounded and adjustable lby dial 11. Here again, a resistor 28 Vis shunted .across the ungrounded end to the wiper on resistor 27 for purposes of load correction. Scale changer 13 `consists .of resistors 29, 30, and 31 serially connected be- '..tween the output of variable inductance 23 and signal are such that adjustable resistance 31 may be preset to` precisely calibrate for a desired ratio, such as 10 to 1, and scale changer 13 thus permits a scalev associated with dial 11 of precision attenuator 10 to be expanded in a predetermined ratio, such as 10 to 1.

In 4channel 5, 90 `phase shifter and visolation device A14 includes a transformer with la primary 33 across the source 6 of voltage and a secondary 34,` and a lagrcircuit comprising Yinductor 35 and variable resistorv 36 having a center tap 18 signal groundedv for phase reversal. This circuit provides a voltage across resistor 36 lshifted out of phase with reference to the source by an angle approaching 90. In order to provide exactly 90 phase shift, ya correction is added by use of a parallel lcircuit across resistor 36, comprising variable resistor 37 and another' secondary winding 38.` The .phasing between secondaries 34 and 38 and the-va1ue of Iresistor 37 are of the product vof angular frequency and inductance di-v vided by the resistance is maintained large for the frequency range over which the instrument is designed to be operated.

Another position associated with gang switch 12 permits the alternate selection in channel 5 of fixed attenua- Y tor and isolation device 15, consisting of transformer 39 and associated pre-set dropping resistor 40, instead of i phase shifter and isolation device 14. When'so connected, the network 15 impresses uponresistor 36 la signal equal in magnitude to that developed by network 14 but in phase with respect to the reference voltage at source 6. Dial 17 associatedrwith resistor 36 functions as previously described to permit adjustment of the voltage output of channel 5, which is introduced as the input to resistor 22 of summing device 1.

The line between test probe 3 and resistor 20 of summing device 1 is interrupted by another blade of gang switch `12. In the first or lower position of gang switch 12, as shown in Figure 5, the summing device receives the probe input. 4 is labeled 0 phase to indicate that the component of the signal from the probe which is nulled under these operative conditions is in phase with the reference voltag while in channel 5 network 14 is placed in the circuit. The second switch position, noted at 180 in channel 4, has no effect on the probe input and channei 5 circuits, but displaces Vthe phase l80 in channel 4 by reversing the connections to the secondary Winding of isolation device 7. Thus, a null obtained with switch 12 in this position indicatesv that a component of the probe signal is of opposite phase to the reference signal. Each of the third and fourth positions of gang switch 12 disconnects probe 3 and signal grounds resistor 20, while in channel 5 network 15 is switched in. In channel 4 the functions of the rst two positions are repeated but are labeled 2l-90 and ,-90, respectively, for purposes which are described below. Another' switch 41 may be interposed in the input line of test probe 3 totransfer the signal directly to the oscilloscope to observe lits nature if desired.

In operating the instrument, the reference signal in the form of source 6 is connected to the input terminals of the instrument and X and Y inputs to the null indicator are connected as has been described hereinbefore. The dial 11 of precision attenuator 10 is first turned to its extreme or position, gang switch 12 is placed in the lower position shown, and the terminal G of the signal ground system is tied into the signal ground system of the unit under test. The signal ground system of coursemust eventually return to power ground. Then j probe 3 is touched to the source of the larger signal voltage V1, to introduce that voltage through resistor 20 of summing device 1. The in-phase component Vm `may be nulled against the isolated voltage in channel 4 with Calibrating attenuator 8 by adjusting with coarse and fine dials 9 and 25 as necessary. The null point mayV readily be observed on the oscilloscope screen by the disposition of a Lissajous type visual pattern, but if the in-phase component is out of phase with respect to lthe reference voltage, nulling cannot take place until the isolated voltage in channel 4 is reversed 180 by switch 12 shifting to the pair of taps in the circuit identified 180. Simultaneously with the nulling of the in-phase component, the quadrature phase may be nulled with The same relative position across channelV `attenuator' 16 against the"is'olated"voltage in` chann'e'lS that has been caused to lag 90 by network 14. Nulling of the quadrature component will be indicated on the oscilloscope-screen by the merging of the Lissajous'pattern into aA single straight line. Since attenuator 16 includes a resistor 36 with a grounded center tap 18, phase reversalof the voltage across resistor 36 is accomplished merely by moving the variable contact from one side of the center tap to the other. In this way, by a simple and rapid trial anderror method, quadrature interference of either quadrant'or phase sense may jbe eliminated to permit precise attenuation of'the reference voltage in Vchannel 4 with tine adjustment dial A25. At the null point 'the voltage appearing across attenuator 8 is now established as the modied referencervoltage, vwhich is equal in magnitude and opposite in phase to the in-phase component of V1, Vpl, while the output voltage of channel 5 is` equal in magnitude to Vgl ybut is opposite in phase.

Probe'3 is next removed from the V1 tap and is placed qu the V2 signal to introduce the second signal'to surnniing device 1. yIt will be recalled that precision at- 'tenuator' v10 was irst adjusted to its maximum or 100% position ,by dial 11. The in-phase component Vpz may nowbe nulled with dial 11, and through a scale associated therewith, va direct reading in percent of the ratio V92/Vm may be had, the accuracy of which depends uponthe calibrated accuracyV of attenuator and stability of other components. If the phase is opposite to the reference, then switch 12 may-be shifted to the 180 Aposition to permit nu'lling, this shift indicating phase sense of Vpz. The quadrature compenent Vqz may be nulled with dial 17, as before, though the circuit shown also'allows' the ratio Vqz/Vpl to be measured by lshifting 'gang lswitch 12 to its third position to disconnect the probe, signal ground resistor 20, and connect alternate Ynetwork 15 in channel 5. At this settingthe only inputs to` summing device 1 are the voltages from channels 4 and S. A voltage equal in magnitude to Vqz but in phase with the reference source', is produced by network 15 and alternator 16 and developed across resistor 22. Accordingly, it is possible to obtain a balance or null by lattenuating the modied reference voltage with dial 11. If` Vqz is leading the reference voltage by 90, this condition is indicated by the +90 switch position. If Vqg .lags by 90, the -90 switch position is used to reverse the voltage in channel 4 before nulling. The switch position again indicates the relative phase sense of Vqg, and setting of dial 11 furnishes directly the percentage Obviously, the ratio Vql/Vpl could have been determined in a similar manner.

l, A panel board for the instrument `described above is shown in Figure 6. The oscilloscope is connected across the two taps in the lower right hand corner designated Y axis and X axis. The signal ground system of the unit to be tested is plugged into the signal ground system of the present instrument through the GND tap at the lower n'ght hand corner. The test probe plugs into the center Probe tap.` Power input is through the two taps in the lower left hand corner marked 115 v. 400- and GND, this instrument having been designed for 115 volts and a frequency or" 400 cycles.

The upper left dial marked Coarse Calibration corresponds to dial 9, Figures 1 and 5, while the dial below that labeled Fine Calibration is dial 25. Each has appropriate scale markings. Precision attenuator 10 is adjusted with the large center dial labeled Ratio Dial, having an outer scale with readings from 0 to 1000 units by hundreds and a concentric inner dialscaled with readings from 0 to 100 units, the two dials being geared 10 to 1. Scale expansion is accomplished with the knob labeled Dial Multiplier, with positions for full scale and one-tenth scale marked XL and X.l," respectively. This corresponds to switch 32 in Figure 5. i'Dial V17 for attenuating either the quadrature voltage developed by 90 Vphase shifter and isolation device 14,

the circuit from the probe is through the summing Vnetv work, and the other setting marked Direct when the input is to oscilloscope.

This completes an instrument with manual control for comparing two voltages in a rapid and accurate manner.

The device may also ybe designed for automatic operation by replacing the oscilloscope with a servomechanisrn coupled to the attenuators in channels 4 and 5 and responsive to the output of the summing device.l Such a scheme is illustrated in Figure 2 Where circuit elements represented in Figure 5 are similarly numbered. It may be noted in this connection that isolation transformer 7 is interposed between precision attenuator 10 and calibrating attenuator 8 and that the ine adjustment on Calibrating attenuator 8 is eliminated. These differences over the arrangement in Figure 5 are questions of convenience of design. Another difference is that the system `of Figure 2 employs a transformer havin a grounded center tap 7a on the secondary winding for phase reversal in channel 4. i

The servomechanism includes a servoamplilier 42, the input to which is output 2 of summing device 1, and a pair of'two phase induction type servomotors 43 and 44 for channels 4 and 5, respectively. The servomotors are connected in parallel to receive the output of servoamplier 42 and are coupled respectively to gear reduction units 45 and 46 leading to the adjustable elements of attenuators 10 and 16 respectively. The second Winding of servomotor 43 receives its excitation from a motor reference source 47 connected to the source 6 of reference voltage. Motor reference source 47 shifts the phase of the reference voltage while servoampliiier 42 may be designed for either no phase shift or complete reversal, so that servomotor 43 operates only when the input from servoampliiierv 42 'includes an inphase component or one shifted thereto.

The excitation of servomotor 44 is from motor reference source 48 that likewise connects the source 6 of reference voltage; however, motor reference source 48 is essentially a transformer and does not shift the reference phase; hence, if the input to servomotor 44 from servoamplier 42 contains' quadrature, servomotor 44 will operate.

The motor reference sources may vinclude inductance,. resistance or capacitance and a vacuum tube amplifier, if? high impedance is required. A more complete descrip-- tion of servomotor systems including the usual stabilizers; and their operation appears in copending application for United States Letters Patent Serial Number 226,861, filed May 17, V1951, by John G. Urbanik. i i

A gang switch 49 has two selectable positions to connect in channel 5 either 90 phase shifter and isolationt device 14 or fixed attenuator and isolation device 15'., At the Sametime, gang switch 49, when in the position illustrated, connects probe 3 to resistor 20 and connects; motor reference'source 48 to the line, but inthe alternate position it signal grounds both resistor 20 and motor Vref'- erence source 48.

To use the instrument just described, test probe 3 is.V

the modified reference voltage in effect is stored in chan nel 4 and, if the ratio Vql/Vm is desired, switch 49 is changed Vto its alternate position so that the only inputs to `summing device' 1 arey from channels 4 and 5, since resistor is signal grounded. At the same position, the excitation on the second windingof servomotor d is signal grounded to prevent possible movement of its rot-or whichWould alter the null setting of attenuator 16 and fixed attenuator and iso-lation device l5 is in the "f cuit. A null effected with servomotor 43 then. pro a direct reading ondial li of the ratio K11/V51.

To measure the relations of the second signal voltage V2, switch 49 is returned to its first position and probe 3 is touched to the V2 tap. The new input through sum ming device l will cause the servornec'nanism to ad,L attenuators 1d and 't6 as necessary to reach another nun. At this` time, dial lli will indicate directly the ratio V92/Vm'. If switch r9 is now changed to its lower position, another null is obtained similarly and the reading on dial 11 will be equal to the ratio Vqz/i/m.

For some applications, it may be desirable to have direct and simultaneous readings of the ratios VpZ/Vpl and Vqz/Vpl. Apparatus for accomplishing this is illustrated by Figure 3, which is a modification of the instrument in Figure V2 in the following respects. Channel Sis in parallel with the output of Calibrating attenuator 3 in channel 4. Fixed attenuator and isolation device l5 is eliminated as is gang switch 459. The same type of servomechanismloops may be used in the embodiments of Figures l aud 5. cision type like attenuator itil and has a dial Yla for direct ratio reading.

In using this instrument, probe 3 is touched to the tap of the first signal voltage at which condition the servomechanism nulls each V1 component as before. Calibrating attenuator S is then adjusted manually until dial 11 is at its 100% position. The output of calibrating attenuator 8 is the input to each of channels fl and 5. The transfer' ratios of channels d and 5 are established as identical by use or" trim resistor 37a. With the zero reading of dial 17a representing no voltage output from impedance 36a, dial la will here fui .ish directly the ratio Vgl/Vm. 90 phase shifter and isola .on device 14 produces a constant attenuation with the result that voltage changes in channel 5 do not alter the presentation of percentage reading on dial Ha.

Now when probe 3 is connected to V2, the servomechanism, through servomotors d3 and da, nulls the two components, and dials il and 17a provide directly the ratios Vlg/V91 Vand VGZ/Vgl, respectively.

A fourth instrument is shown in Figure 4 for using the rst signal voltage as a lreference and measuring the ratios V92/V1 and Vqz/ V1. The reference voltage input is through a second test probe 3a. In-phase channel 4 comprises an isolation amplier 50 and precision attenuator 10, while quadrature channel 5 includes a similar isolation amplifier 51, 90 phase shifter and isolation device 14 and precision attenuator 16a. These two channels furnish inputs to summing device 1 as does a test circuit through probe 3. The isolation ampliiiers are feedback `ampliiers which provide output voltages the same in magnitude and phase as input voltages, and are characterized by very high input impedance and low output impedance. The circuit shown in Figure 4 is for manual operation, as in Figure 5, but it could be modi lied for servo operation.

In using this device, probe 3a is connected to V1 and probe 3 to V2. Dials 11 and 17a may then be adjusted manually to null out both the in-phase and quadrature components of V2 with respect to V1. With dials 11 and 17a calibrated with their Zero points at no output of impedances 27 and 36a, respectively, readings at the null point furnish the ratios VPE/V1, and Vqg/ V1. The'characteristics of V1 with respect to the common reference In addition attenuator la is a pre'` to the output of .said servoamplier, said servomotors` Vsource may be determined in similar manner by touching probe 3 to V1 and connecting probe 3a to the source.

We claim: 1 Y u p 1. Electrical measuring apparatus compnslng a Ysumming device,

the summing device, means for impressing on said sumj ming device a voltage externally derived from the refer,- ence voltage, and a servo system includinga servoampliiier having an input connected to the output of the stunming device, first and second servomotors connected having rotors, rst gear means coupling the rotor of a first of said motors to the indicating means associated with the? second invphase attenuatons'econd gear means coupling the rotor of the second of said motors to the mdicatmg means of said quadrature attenuator,A iirstphase-s'liift means coupling said second voltage to said rst servo# t motor and second phase-shift means coupling said second voltage to said second servomoton said rst phase-shift means being adapted to'furnish an output voltage 90 displaced with respect to the in-phase component of said first voltage at the output of -said servoamplilier, and said second phase-shift means being adapted to furnish an out'- put voltage displaced with respect to the quadrature t component of said rst voltage at the output of said servot ampliiier. j p

2. An instrument for measuring the in-phase and quadrature components of a signal relative to a synchronous source comprising means for developinga irst signal Vin l phase therewith, rst adjustable attenuation means and` a calibrated attenuation means serially connected to re'- ceive said rst kin-phase signal, means coupled to saidV source for developing a signal in quadrature therewith,"

means coupled to said source for developing a second signal in phase therewith and having an amplitude equal 'to said quadrature signal, second adjustable attenuation means connectibly disposed to receive either of said two last-named signals, summing means connected to receive the signal to be measured and the output signals of said rst and second adjustable attenuation means, means forV selectively switching the input of said second attenuation means from said quadrature'signal to said second inphase signal, said last-mentioned means being adapted to simultaneously disconnect said signal to be measured from said summing means, and a phase-sensitive means con-` nected to indicate the difference between the output of said summing means and said source.

3. An instrument for measuring the m-phase and quad-4 rature components of a signal relative to a synchronous source comprising means for developing a first signal in phase therewith, iirst adjustable attenuation means and a calibrated attenuation means serially connected to re' ceive said rst in-phase signal, means coupled to saidl source for developing a signal in quadrature therewith,

means coupled to said source for developing a second Y signal in phase therewith and having an amplitude equal to said quadrature signal, second adjustable attenuation means connectibly disposed to receive either of said two last-named signals, summing means connected to receive the signal to be measured and the output signals of said` rst and second attenuation means, means for selectively switching the input of said second attenuation means from said quadrature signal to said second in-phase signal, said last-mentioned means being adapted to simultaneously dis-V l connect said signal to be measured from said summing means, and null indicator means. vconnected to indicate the difference between the output of said summing means and said source.

4. An instrument for measuring the in-phase and quadrature components of a signal relative to a synchronous, y

an` in-phase channel adapted tol impress. a reference voltage on said summing dev1ce,usa1d inl-phase y source comprising means for developing a rst signal-in phase therewith, said means including selective phasereversal means, first adjustable attenuation means and calibrated attenuation means serially connected to receive said first in-phase signal, means coupled to said source for developing a signal in quadrature therewith, means coupled to said source for developing a second signal in phase therewith and having an amplitude equal to said quadrature signal, second adjustable attenuation means connectibly disposed to receive either of said two lastnamed signals, summing means connected to receive the signal to be measured and the output signals of said rst and second attenuation means, means for selectively switching the input of said second attenuation means from said quadrature signal to said second in-phase signal, said last-mentioned means being adapted to simultaneously disconnect said signal to be measured from said summing'means and null indicator means connected to indicate the dierence between the output of said summing means and said source.

5. An instrument for measuring the in-phase and quadrature components of a signal relative to a synchronous source comprising means coupled to said source for developing a first signal in phase therewith, a first adjustable attenuation means and a calibrated means serially connected to receive said in-phase signal, means coupled to said source for developing a signal in quadrature therewith, means coupled to said source for developing a second signal in phase therewith and having an amplitude equal to said quadrature signal, second adjustable attenuation means connected to receive said two lastlast-mentioned means being operative upon switching to said latter position to simultaneously disconnect said signal to be measured by said summing means, and a null indicator means connected to indicate the diierence of the output of said summing means and said source.

6. An electrical measuring instrument in accordance with claim in which the means developing the quadrature signal includes selective phase-reversing means.

7. An electrical measuring instrument in accordance with claim 5 in which the means coupled to said source are electrically isolated therefrom.

8. Electrical measuring apparatus comprising a summing device, means for connecting a null indicator to said summing device, means for impressing on'said summing device a Voltage externally vderived from a reference voltage, an in-phase channel adapted to impress Vthe reference voltage on said summing device, said n-phase channel including a Calibrating attenuator and a second attenuator having indicating means associated therewith, and a quadrature channel adapted to shift the phase of the reference voltage and to impress the phase shifted voltage on the summing device, said quadraturechannel including a voltage attenuating means, means shifting the phase of the reference voltage by 90, means for selectively impressing on the quadrature channel attenuating means either the Aoutput of the phase'shifting means or a voltage having the same magnitude as and 180 out of phase with the reference voltage, and a v 10 secondof said motors to the adjustable memberlof s'id quadrature attenuator, iirst phase-shiftv meansrcoupling said second voltage to said tirst servomotori and second phase-shift means coupling with second voltageto said second servomotor, said rst phase-shift means Vbeing adapted to furnish an loutput voltage 90 displaced with respect-'to the in-phase'component of said rst voltage put impedance, rst adjustable attenuation means aridA a calibrated attenuation means serially connected to receive said first in-phase signal, means coupled to said source for developing a signal in quadrature therewith, said means including an isolation amplifier having a high input impedance means coupled to said source for developing the second signal in-phase therewith and having an amplitude equal to said quadrature signal, second adjustable attenuation means connectibly disposed to receive either of said two last-named signals, summing means connected to receive the signal to be measured and the output signals of said rst and second attenuation means, means for selectively switching the input of said second attenuation means from said quadrature signal to said second in-phase signal, said lastmentioned means being adapted to simultaneously disconnect said signal to be measured from said summing means and null indicator means connected to indicate the difference between the output of said summing means and said source.

l0. An instrument for measuring the in-phase and quadrature components of a signal relative to a synchronous source, means comprising said source for developing a rst signal in phase therewith, -rst adjustable attenuation means and calibrated attenuation means serially connected to receive said first in-phase signal, means coupled to said source for developing a signal in quadrature therewith, said means including a phase-shift lag network, means coupled to said source for developing a second signal in phase therewith and having an amplitude equal to said quadrature signal, second adjustable attenuation means connectibly disposed to receive either servo system including a servoamplier having an input of said two last-named signals, summing means connected to receive the signal to be measured and the output signals of said first and second attenuation means from said quadrature signal to said second in-phase signal, said means being adapted to simultaneously disconnect said signal to be measured from said summing means and null indicator means connected to indicate the difference between the output of saidy summing means and said source.

ll. An electrical measuring instrument in accordance with claim 10 in which said lag network consists of inductance and resistance elements serially connected.

l2. An electrical measuring instrument in accordance with claim ll in which the parameters of the inductance 1 and resistance elements of the lag network minimize 11 V5;,lironcnis-A A source comprising means for developing a first signal in 'phase therewith, said means including selective phasefreyersal means, rst adjustable attenuation means and calibrated attenuation means serially connected to receivesaid in-phase signal, means coupled to said source oi'developing a signal in quadrature therewith, said g viirst and second attenuation means, means for selectively switching the input of said second attenuation means from said quadrature signal to said second in-phase signal, said last-mentioned means being adapted to simultaneously disconnect said signal to be measured from said surnming means and-a phase-sensitive means connectedto indicate thediffrence bretweenthe output of said surnminggmeans and said source. Y

l i "Rete're'nces Cited in the le of this patent Y UNITED STATES PATENTSV 2,440,200,r r10J-fell Apr. 20, 1948 l 2,595,675 Jaynes May 6, 1952 2,618,686 Lange Nov. 18, 1952 2,622,127` Alsbeig Dec. 16, '1952 Y2,639,411 V'Schafer May 19, 1953 v2,657,348 Jarvis oct. 27k, 1953 OTER REFERENCES l, g Electronic instruments, Greenwood, Holdam, MacRae,

McGraw-'Hill Book Co., 1948, page' 33. (Copy 'in Div. i 

